The field of fiber optics includes the technique of transmitting light through long, thin, flexible fibers of glass, plastic, or other transparent material. In the field of integrated fiber optics, the problem of precisely coupling fibers to integrated optical chips still demands a satisfying solution, especially with regard to long-term stability and economic aspects. Alignment of single-mode fibers to optic devices has proven to be the most expensive single item in the cost of packaged photonic devices, particularly where the desired packaged photonic device is hermetic. The present invention discloses a device and method which solves problems associated with the coupling of fibers to integrated optical chips.
Two prior art techniques attempt to solve these problems. The first technique involves coupling optical fibers to an optical chip by manual or semi-automatic adjustment, along with attaching the optical fibers to the optical chip with optical adhesives. The second technique involves prealigning optical fibers in an array. This optical fiber array can be constructed, for example, by anisotropically etching V-grooves in a silicon substrate, and adjusting and fixing the array to match the integrated optical component.
Nonetheless, both methods have severe disadvantages. Adhesives have thermal expansion coefficients far greater than that of optical chip and glass fibers. Also, at high temperatures, part of the adhesives may out-gas and damage the integrated optical circuit. With respect to the use of silicon V-groove arrays, there are also problems caused by thermal expansion. This is particularly true in the case of large arrays that have distances of several millimeters between the first and last fibers because the thermal expansion coefficient of silicon and optical chip materials like glass or LiNbO.sub.3 are very different from one another. An additional problem associated with the use of silicon V-groove arrays is the high expenditure of time that results from these manual alignment techniques.
Devices are known for positioning optical fibers so that an axis of a fiber is positioned with respect to a reference axis. Representative of such devices is that shown in U.S. Pat. No. 4,756,591 (Fischer et al.), wherein a V-groove is formed in a silicon substrate and an elastomeric member is biased against the fiber to hold it in the groove. The groove may be stepped to provide a deeper groove segment to hold the jacket of the fiber within the device.
U.S. Pat. No. 4,756,591 (Sheem) discloses a grooved silicon substrate having a pair of intersecting V-grooves therein. A fiber to be positioned is disposed in one of the grooves while a shim is disposed in the other of the grooves. The shim may take the form of a tapered or an eccentric fiber, which when respectively slid or rotated under the first fiber serves to lift the same to bring the axis thereof into alignment with a reference axis. A cover may be positioned above the substrate to assist in clamping the first fiber into position.
U.S. Pat. No. 4,802,727 (Stanley) also discloses a positioning arrangement for optical components and waveguides which utilizes a V-grooved structure. U.S. Pat. No. 4,862,272 (Pimpinella et at.) and U.S. Pat. No. 4,830,450 (Connell et al.) discloses arrangements for positioning an optical fiber that utilize members having frustoconical apertures therethrough.
Optical fibers generally include an inner core with a predetermined index of refraction. This inner core is surrounded by a cladding layer. The cladding layer has an index of refraction lower than that of the inner core. The inner core is the medium in which the optical energy is guided, while the cladding layer defines the index boundary with the core. The outer diameter of the fiber may vary in dimension about a predetermined nominal dimension. It has been seen, for example, that two nominally identical fibers from the same manufacturer may vary in outside diametrical dimension by as much as +4 .mu.m to -4 .mu.m. This fiber to fiber variation in outer diameter makes difficult the accurate positioning of the axis of the core of a fiber with respect to a predetermined reference axis using a positioning apparatus having a V-grooved structure.
Other methods in the prior art for positioning fibers include that disclosed in U.S. Pat. No. 5,243,673 (Johnson et al.) which describes a positioning apparatus that aligns a predetermined point on an optical fiber with a reference axis. Johnson et al shows an opto-electronic component having a positioned optical fiber associated therewith. Johnson includes an opto-electronic device of either an edge or surface active type mounted on a pedestal.
The structure and method disclosed according to the present invention is different from the foregoing references and other devices and techniques in the prior art, and offers many advantages over the prior art. An advantage of the present invention is that it discloses a Z-axis approach, unlike Johnson or the aforementioned prior art. In a z-axis approach, fibers are brought in normal to the surface, whereas with the aforementioned prior art, including Johnson, the fibers are positioned parallel to the surface. As a result, the prior art utilizes an approach involving butt-coupling for optical devices, or a turning-mirror at the end of a channel to re-direct light up or down. The aforementioned prior art patents also utilize V-grooves which slide the fiber along, effectively tapering the fiber and placing pressure on the sides of the fiber. Another advantage of the present invention is that it may be entirely independent of lenses. Still another advantage of the present invention is that it employs a device and concept for capturing fibers that may be applied to lasers, detectors, gratings, or other optical output or input devices, which affords a wide range of practical applications in the area of fiber alignment for opto-electronic devices.